The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations, and of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutting structures mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit body for cooling and/or cleaning the cutting structures.
In a common form of such a drill bit the cutting structures comprise so-called "preform" cutting elements. Each cutting element is in the form of a tablet, usually circular or part-circular, having a hard cutting face formed of polycrystalline diamond or other superhard material. Normally, each such preform cutting element is formed in two layers: a hard facing layer formed of polycrystalline diamond or other superhard material, and a backing layer formed of less hard material, such as cemented tungsten carbide.
In one commonly used method of making rotary drill bits of the above mentioned type, the bit body is formed by a powder metallurgy process. In this process a hollow mould is first formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with powdered material, such as tungsten carbide, which is then infiltrated with a metal alloy binder, such as copper alloy, in a furnace so as to form a hard matrix. The maximum furnace temperature required to form the matrix may be of the order of 1050.degree. to 1170.degree. C. Conventional two-layer preforms of the kind described, however, are only thermally stable up to a temperature of 700.degree. to 750.degree. C. For this reason preform cutting elements are normally mounted on the bit body after it has been moulded. There are, however, now available polycrystalline diamond materials which are thermally stable up to and beyond the range of infiltration temperatures referred to above. Such thermally stable diamond materials are, for example, supplied by the General Electric Company under the trade name "GEOSET" and by De Beers under the trade name "SYNDAX 3".
These materials have been applied to matrix-bodied bits by setting pieces of the material in the surface of a bit body so as to project partly from the surface. The pieces have been, for example, in the form of a thick element of triangular shape, one apex of the triangle projecting from the surface of the drill bit and the general plane of the triangle extending either radially or tangentially. Means have also been devised for mounting on matrix-bodied bits thermally stable elements of similar configuration to the non-thermally stable two-layer elements of the kind previously described, for example elements in the form of circular tablets. Arrangements and methods for mounting such thermally stable cutting elements on matrix bodied bits are described in U.S. Pat. No. 4,624,830.
Although such thermally stable preform cutting elements are of obvious application to matrix bodied bits, since they may be incorporated in the surface of the bit body during the process of moulding the bit body, the present invention is based on the application of thermally stable preform cutting elements to drill bits where the bit body is formed from steel.